Class 12 Chemistry · Chapter 10 NotesBiomolecules

Biomolecules are the organic compounds that make up living systems and carry out the chemical reactions of life. The main classes are carbohydrates, proteins, nucleic acids, lipids, vitamins and hormones. Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. They are classified by their behaviour on hydrolysis into monosaccharides (cannot be hydrolysed further, e.g. glucose, fructose, ribose), oligosaccharides (give two to ten monosaccharide units, e.g. sucrose, maltose, lactose) and polysaccharides (give a large number of units, e.g. starch, cellulose, glycogen). Carbohydrates that reduce Fehling's solution and Tollens' reagent are called reducing sugars; all monosaccharides are reducing sugars. Monosaccharides are further named by the number of carbon atoms and the functional group: an aldose has an aldehyde group and a ketose has a keto group, giving terms such as aldotriose, ketohexose and so on.

• Monosaccharidesglucose, fructose, ribose
• Oligosaccharidessucrose, maltose, lactose
• Polysaccharidesstarch, cellulose, glycogen
• Reducing sugars reduce Fehling's solution and Tollens' reagent

Glucose is an aldohexose, also called dextrose, and is the most abundant organic compound on earth. It is prepared by boiling sucrose with dilute HCl or H2SO4, or by hydrolysis of starch with dilute H2SO4 at 393 K under pressure. Its molecular formula is C6H12O6. Evidence for its open chain structure includes: reaction with hydroxylamine to form an oxime and with HCN to form a cyanohydrin (carbonyl group present); oxidation with bromine water to gluconic acid (aldehyde group present); acetylation to glucose pentaacetate (five –OH groups); and oxidation with nitric acid to saccharic acid (primary alcoholic group). Glucose is D(+)-glucose. The open chain structure cannot explain why glucose does not give Schiff's test, why its pentaacetate does not react with hydroxylamine, and why it exists in two crystalline forms (α and β). These facts are explained by a cyclic hemiacetal structure in which the –OH at C-5 adds to the –CHO group, forming a six-membered pyranose ring. The two forms differ at C-1, the anomeric carbon, and are called anomers. Fructose is a ketohexose, D(–)-fructose, found in fruits and honey; it forms a five-membered furanose ring.

• Glucosealdohexose, C6H12O6, D(+)-glucose
• Fructoseketohexose, D(–)-fructose
• Cyclic formsα and β anomers; pyranose for glucose, furanose for fructose

Disaccharides are formed when two monosaccharide units join through an oxide linkage called a glycosidic linkage, with the loss of a water molecule. Sucrose gives D-(+)-glucose and D-(–)-fructose on hydrolysis; the linkage is between C1 of α-D-glucose and C2 of β-D-fructose, so both reducing groups are involved and sucrose is a non-reducing sugar. Sucrose is dextrorotatory, but after hydrolysis the mixture is laevorotatory because fructose is more laevorotatory than glucose is dextrorotatory; this product is called invert sugar. Maltose contains two α-D-glucose units linked C1 to C4 and is a reducing sugar. Lactose (milk sugar) is made of β-D-galactose and β-D-glucose linked C1 to C4 and is also reducing. Polysaccharides contain many monosaccharide units joined by glycosidic linkages. Starch is a polymer of α-glucose and has two components: amylose (water soluble, unbranched, C1–C4 linkages, about 15–20%) and amylopectin (insoluble, branched, C1–C4 chains with C1–C6 branching, about 80–85%). Cellulose is a straight chain polymer of β-D-glucose units linked C1 to C4 and is the most abundant organic substance in plants. Glycogen is the animal storage carbohydrate, similar to amylopectin but more highly branched, found in liver, muscles and brain.

• Glycosidic linkagebond between two monosaccharides through oxygen
• Sucrosenon-reducing; maltose and lactose: reducing sugars
• Starchamylose + amylopectin; cellulose: β-D-glucose chains; glycogen: animal starch

Proteins are polymers of α-amino acids linked by peptide bonds. Amino acids contain both –NH2 and –COOH groups; only α-amino acids are obtained on hydrolysis of proteins. They are classified as acidic, basic or neutral depending on the relative number of amino and carboxyl groups. Essential amino acids cannot be synthesised in the body and must come from the diet; non-essential amino acids can be synthesised. In aqueous solution, the carboxyl group loses a proton and the amino group accepts one, forming a dipolar ion called a zwitter ion, which makes amino acids amphoteric. Except glycine, all naturally occurring α-amino acids are optically active and most have L-configuration. A peptide bond (–CO–NH–) is an amide linkage formed between the –COOH of one amino acid and the –NH2 of another, with elimination of water. Two amino acids give a dipeptide, three a tripeptide, and many give a polypeptide; a polypeptide with more than about hundred amino acid residues and molecular mass above 10,000 u is called a protein. Proteins are fibrous (parallel chains, insoluble in water, e.g. keratin, myosin) or globular (coiled chains, soluble, e.g. insulin, albumins). Protein structure has four levels: primary (sequence of amino acids), secondary (α-helix and β-pleated sheet, stabilised by hydrogen bonds), tertiary (overall folding, stabilised by hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces) and quaternary (spatial arrangement of subunits). Denaturation is the loss of biological activity when hydrogen bonds are disturbed by heat or pH change; secondary and tertiary structures are destroyed but the primary structure remains intact. Boiling an egg coagulates the egg white by denaturation.

• Peptide bond–CO–NH– formed by elimination of water
• Zwitter iondipolar ion giving amphoteric behaviour
• Fibrous proteinskeratin, myosin; globular proteins: insulin, albumins
• Denaturationsecondary and tertiary structures destroyed, primary intact

Enzymes are biocatalysts that speed up the chemical reactions of life under very mild conditions. Almost all enzymes are globular proteins. They are highly specific for a particular reaction and a particular substrate. Enzymes are generally named after the compound or class of compounds on which they act, such as maltase, which catalyses the hydrolysis of maltose into glucose: C12H22O11 + H2O → 2 C6H12O6. Some enzymes are named after the reaction they catalyse, for example oxidoreductases catalyse oxidation of one substrate with simultaneous reduction of another. The name of an enzyme ends in -ase. Enzymes are needed only in small quantities. Like chemical catalysts, they reduce the magnitude of activation energy; for example, the activation energy for acid hydrolysis of sucrose is 6.22 kJ mol⁻¹, while with the enzyme sucrase it is only 2.15 kJ mol⁻¹.

• Enzymes are mostly globular proteins and are reaction-specific
• Enzyme names end in -ase
• Enzymes lower activation energy

Vitamins are organic compounds required in small amounts in the diet for normal growth, health and specific biological functions. Most vitamins cannot be synthesised in the body, though plants can synthesise almost all of them and gut bacteria produce some. Vitamins are classified by solubility. Fat-soluble vitamins A, D, E and K are stored in liver and adipose tissue. Water-soluble vitamins include the B group and vitamin C; they must be supplied regularly because they are readily excreted in urine and cannot be stored (except vitamin B12). Deficiency diseases include: vitamin A – xerophthalmia and night blindness; vitamin B1 (thiamine) – beri beri; vitamin B2 (riboflavin) – cheilosis and digestive disorders; vitamin B6 (pyridoxine) – convulsions; vitamin B12 – pernicious anaemia; vitamin C (ascorbic acid) – scurvy; vitamin D – rickets in children and osteomalacia in adults; vitamin E – increased fragility of RBCs and muscular weakness; vitamin K – increased blood clotting time. Excess of vitamins is also harmful.

• Fat-solubleA, D, E, K (stored in liver and adipose tissue)
• Water-solubleB group and C (excreted in urine, need regular supply)
• Deficiency diseasesscurvy (C), rickets (D), beri beri (B1), night blindness (A)

Nucleic acids are long chain polymers of nucleotides and are responsible for the transmission of hereditary characters. Complete hydrolysis of DNA or RNA gives a pentose sugar, phosphoric acid and nitrogen-containing heterocyclic bases. DNA contains β-D-2-deoxyribose and the bases adenine (A), guanine (G), cytosine (C) and thymine (T). RNA contains β-D-ribose and the bases A, G, C and uracil (U) in place of thymine. A base attached to the 1′ position of a sugar is a nucleoside; when the nucleoside is linked to phosphoric acid at the 5′ position, it becomes a nucleotide. Nucleotides are joined by phosphodiester linkages between the 5′ and 3′ carbon atoms of the pentose sugar. The sequence of nucleotides is the primary structure. Watson and Crick proposed the double strand helix structure for DNA, in which two chains are wound about each other and held by hydrogen bonds between specific base pairs: adenine pairs with thymine, and cytosine pairs with guanine. The two strands are therefore complementary. RNA is single stranded and sometimes folds back on itself; it occurs as mRNA, rRNA and tRNA. DNA is the chemical basis of heredity, can self-duplicate during cell division, and carries the coded message for protein synthesis, which is actually carried out by RNA. DNA fingerprinting uses the unique base sequence of an individual and is used in forensic identification, paternity testing and identification of accident victims.

• Nucleosidebase + sugar; Nucleotide: base + sugar + phosphate
• DNA basesA, G, C, T; RNA bases: A, G, C, U
• Base pairingA–T and C–G in DNA
• RNA typesmRNA, rRNA, tRNA

Hormones are molecules that act as intercellular messengers. They are produced by endocrine glands and poured directly into the blood stream, which transports them to the site of action. Chemically, some hormones are steroids (estrogens, androgens), some are polypeptides (insulin, endorphins) and some are amino acid derivatives (epinephrine, norepinephrine). Hormones maintain the balance of biological activities. Insulin keeps blood glucose within narrow limits and is released when blood glucose rises rapidly; glucagon increases blood glucose, and the two together regulate glucose level. Epinephrine and norepinephrine mediate responses to external stimuli. Growth hormones and sex hormones control growth and development. Thyroxine, an iodinated derivative of tyrosine produced in the thyroid gland, regulates metabolism; low thyroxine causes hypothyroidism (lethargy, obesity) and high thyroxine causes hyperthyroidism. Low iodine in the diet can lead to hypothyroidism and enlargement of the thyroid gland, which is controlled by using iodised salt. Steroid hormones from the adrenal cortex include glucocorticoids (control carbohydrate metabolism, modulate inflammation, stress response) and mineralocorticoids (control excretion of water and salt by the kidney). Gonadal hormones include testosterone (male secondary characteristics), estradiol (female secondary characteristics and menstrual cycle) and progesterone (prepares the uterus for implantation of the fertilised egg).

• Hormonesintercellular messengers secreted by endocrine glands
• Insulin and glucagon regulate blood glucose
• Thyroxine is an iodinated tyrosine derivative
• Steroid hormonesglucocorticoids, mineralocorticoids, testosterone, estradiol, progesterone